The invention generally relates to multi-purpose welding power supplies for providing different modes of welding capability from a single unit. In addition, there are certain features which enable or enhance the use of a particular mode independently of the others comprising the multi-purpose feature in the welding power supply.
In many welding operations it may be necessary to have the ability to weld utilizing more than one mode of operation. For example, in high speed sheet metal work with consumable electrodes direct current welding is preferred, while in welding softer metals such as aluminum with non-consumable electrodes, alternating is preferred. In the latter instance a constant potential direct current would cause either the work product or the electrode to become excessively hot. An example of such a system is disclosed in the U.S. Pat. No. 2,757,296 to Bishel issued July 31, 1956. Although alternating current and direct current capability have been provided before a single unit, these systems have been characterized at least in part by inefficiencies and complexities which detract from their use in multi-purpose welding units.
Another type of welding known as pulse arc welding, where an axial spray from a consumable electrode occurs at a high current density, has a number of advantages over constant potential or alternating current systems discussed above. Advantages of axial spray pulse arc welding include better direction control in several planes without affecting arc behavior, enhanced penetration due to the concentration of energy of the arc, and the kinetic energy of the metal drop striking the weld pool to produce deep weld penetration. However, in out-of-position welding, the high fluid densities of spray pulse arc welding may be too fluid to control. One approach suggested in overcoming this problem is disclosed in U.S. Pat. No. 3,071,680 to Anderson issued Jan. 1, 1963. Anderson in this patent states that welding current should be supplied in discrete pulses against a background of "keep alive current" which is sufficient to maintain the arc without causing the transfer of metal to the workpiece. This is accomplished by producing current pulses which terminate before a globule of molten metal can form during successive spray producing current pulses. The circuit employed to limit this pulse arc time is excessively complex, employing a number of tubes and inductors rendering the device one which lacks accuracy and is excessively power consumptive as well.
The invention described herein overcomes many of the limitations of multi-purpose power sources for welding discussed above. A single unit comprises circuitry to provide for five different welding modes from a three phase alternating current source through a three (3) stage transformer system to two positive output terminals. The first mode is a standard constant potential welding mode with short arc capability up to 300 amps with fixed slope resistor and variable inductor at the first output terminal. The second mode is a standard constant potential operation for high quality cored wire and mig spray arc welding up to 600 amperes at the second output terminal. The circuitry for the 600 ampere mode bypasses the inductor and slope resistor and utilizes only the slope resistance inherent in the three stage transformer system. The third mode provides pulse arc capability at 60 pulses per second (pps) with a "hot start" feature; the fourth mode also provides pulse arc capability at 60 pps but without "hot start"; and the fifth mode provides pulse arc capability at 120 pps.
The pulses for the third, fourth, and fifth modes are generated with a constant voltage transformer and subjected to half wave rectification to provide sixty (60) pulses of direct current per second or to a full wave bridge rectifier to provide 120 pulses per second. A hot start feature utilized with the 60 pulses per second (pps) pulse welding mode is accomplished by energizing the 120 pps pulse arc mode for a relatively short period of time after which the "hot start" circuitry becomes disconnected from the remainder of the system. This "hot start" is required with 60 pps pulse arc welding to facilitate initiating the arc at lower current levels such as those in the 60 pps mode.
The invention includes a variable pulse width control mechanism to control pulse width for either the 60 pps or 120 pps welding. By properly controlling the pulse width by phase control silicon-controlled rectifiers and a special transformer, a pulse rate can be employed with a sufficient time lapse between the pulses to allow the welding electrode to operate at the proper temperature. The system is particularly adapted for out-of-position welding at a pulse rate of 120 pps.
A feature of the invention includes the ability to perform pulse arc welding at 60 pps, or 120 pps, and standard constant potential welding in a single machine. Other sub-systems which comprise these features noted above are also unique as will be described in detail hereinafter.